Single-mode optical fibers used in telecommunications applications at operating wavelengths near 1310 nm can carry higher order modes in addition to the fundamental mode they are intended to carry. Optical power coupled into these higher order modes at an input end of a fiber are strongly attenuated and are not observed at an output end of the fiber if the fiber is more than a few meters long. However, optical power from both the fundamental mode and higher order modes can be observed at the output end of a fiber shorter than about one meter.
When a short piece of single-mode fiber is connected between a launching fiber and a receiving fiber, most of the optical power of the fundamental mode of the launching fiber is coupled into the fundamental mode of the short fiber. However, some of the optical power of the fundamental mode of the launching fiber is coupled into higher order modes of the short fiber. The fundamental and higher order modes propagate along the short fiber with different propagation delays and reach the junction of the short fiber and the receiving fiber out of phase. Most of the optical power of the fundamental mode of the short fiber is coupled into the fundamental mode of the receiving fiber. Some of the optical power of the higher order modes of the short fiber is also coupled into the fundamental mode of the receiving fiber at the junction of the short fiber and the receiving fiber, where it interferes with optical power coupled from the fundamental mode of the short fiber. The propagation delay difference of the fundamental and higher order modes varies with wavelength, so the optical power of the fundamental mode of the receiving fiber is strongly wavelength dependent. This phenomenon is known as modal interference.
It is generally more convenient to mount an optical fiber pigtail or stub to a connector ferrule or optoelectronic device under ideal factory conditions and to splice that stub to a transmission fiber under less ideal field conditions than it is to mount the transmission fiber directly in the connector ferrule under the less ideal field conditions. However, in this case the optical fiber pigtail or stub is a potential source of modal interference, and the connector insertion loss can be strongly wavelength dependent and therefore practically unpredictable. Short optical fibers may also be found in optical fiber telecommunications systems as patch cords and as pigtails for optoelectronic devices, and these too are potential sources of undesirable modal interference effects, including modal noise.
Optical fiber systems designers have dealt with modal interference problems by using optical fiber having a relatively low cut-off wavelength for pigtails and patch cords. Even so, relatively long lengths of such fiber may be needed to provide the required attenuation of higher order modes, and such lengths must frequently be stored as optical fiber loops.
U.S. Pat. No. 4,877,306 proposes a special fiber design for control of modal noise in short fiber sections. According to this design, an outer cladding layer is selectively doped with FeO, CdO, MO.sub.3, Cr.sub.2 O.sub.3, V.sub.2 O.sub.5, CoO, Nb.sub.2 O.sub.5 or TiO.sub.2 to provide a high refractive index and a high attenuation. This outer cladding layer effectively traps and attenuates high order modes in a few tenths of a meter. This patent specifically teaches away from the use of optical fiber coatings having a higher refractive index than the optical fiber claddings. (U.S. Pat. No. 4,877,306 was issued in the name of Gitimoy Kar on Oct. 31, 1989 and is entitled "Coated Optical Waveguide Fibers".)